Thermal retransfer printing involves forming an image (in reverse) on a retransfer intermediate sheet using one or more thermally transferable dyes. The image is then thermally transferred to a surface of an article by bringing the image into contact with the article surface and applying heat and possibly also pressure. Thermal transfer printing is particularly useful for printing onto articles that are not readily susceptible of being printed on directly, particularly three dimensional (3D) objects. Thermal retransfer printing by dye diffusion thermal transfer printing, using sublimation dyes, is disclosed, e.g., in WO 98/02315 and WO 02/096661. By using digital printing techniques to form the image on the retransfer intermediate sheet, high quality images, possibly of photographic quality, can be printed on 3D articles relatively conveniently and economically even in short runs. Indeed such objects can be personalised economically.
Using suitable retransfer intermediate sheets, it is possible to form good quality images on 3D articles, possibly having complex shapes including curved shapes (concave or convex) including compound curves. When printing onto 3D articles, the sheet is typically preheated, e.g. to a temperature in the range 80 to 170° C., prior to application to the article, to soften the sheet and render it deformable. The softened sheet is then in a condition in which it can be easily applied to and conform to the contours of an article. This is conveniently effected by application of a vacuum to cause the softened sheet to mould to the article. While the sheet is maintained in contact with the article, e.g. by maintenance of the vacuum, the sheet, and also the article, are heated to a suitable temperature for dye transfer, typically a temperature in the range 140 to 200° C., for a suitable time, typically in the range 15 to 150 seconds. After dye transfer, the article is allowed or caused to cool before removal of the retransfer intermediate sheet. Suitable apparatus for performing the retransfer printing step is disclosed e.g. in WO 01/96123 and WO 2004/022354.
Heating of the sheet and article is conveniently effected by exposure to a stream of hot air generated from heating means comprising a fan and electrical heater elements. In known apparatus, the air enters a hood and is directed over articles located in a cavity of the apparatus. In some current systems, air is passed through a diffuser comprising an arrangement of static guides. By careful arrangement of the guides, air distribution over the articles and associated sheet can be achieved that is uniform and optimised, provided the articles have a relatively flat profile and provided the articles are located centrally in the cavity. However, the air distribution is not optimised for articles that have upwardly projecting portions, as side or lower surfaces of the articles tend to remain cooler than upper surfaces, nor for articles located close to the edges of the cavity, as surfaces remote from the centre of the cavity tend to be less well heated. This results in uneven heating of the articles and sheet and consequent variable dye transfer, with potentially poor dye transfer occurring on cooler regions of articles. This can result in poor overall print quality.